Oil-impregnated sintered bearing and method of producing the same

ABSTRACT

An oil-impregnated sintered which does not damage rotating shaft and itself and has a high durability even in the case that the rotating shaft is inclined in the bearing by a large shear load applied thereto, and a method of manufacturing an oil-impregnated sintered bearing which exhibits center deviation-suppressing action of the bearing satisfactorily by accurately forming a bearing hole in an intermediate completely sintered are disclosed.

TECHNICAL FIELD

The present invention relates to an oil-impregnated sintered bearing anda method of manufacturing the same.

BACKGROUND ART

An oil-impregnated sintered bearing includes a sintered body in whichlubricating oil is impregnated in advance, and thermally expands due topumping action and frictional heat caused by rotation of a shaft so thatlubricating oil comes out of the sintered body to lubricate a frictionsurface. Since the oil-impregnated sintered bearing can be used for along time without supply of oil, the oil-impregnated sintered bearinghas been widely used as a bearing for supporting a rotating shaft of avehicle or home electric appliances, an acoustic equipment, etc.

In the conventional oil-impregnated sintered bearing, in order to centerthe rotating shaft inserted into a bearing hole, a portion of thebearing hole is formed to have a diameter smaller than diameters ofother portions in the bearing hole, and the only portion having thesmaller diameter comes into contact with the rotating shaft.

In the meantime, as described above, in a case where a portion of thebearing hole is formed to have a diameter smaller than diameters ofother portions in the bearing hole, since the length of a portionactually contacting the rotating shaft is shorter than the entire lengthof the bearing, there has been problems in that the supporting state ofthe rotating shaft is likely to be unstable and the rotating shaft islikely to be deviated.

Consequently, in the conventional oil-impregnated sintered bearing, thebearing hole is formed to have a journal part that supports the rotatingshaft and enlarged diameter parts that are connected with the journalpart and have constant diameters to be enlarged toward the tips thereof.Furthermore, the enlarged diameter parts are formed to have sinteringdensity higher than that of the journal part in order to suppressdeviation from the center of the rotating shaft (for example, seeJapanese Unexamined Patent Application Publication No. 8-19941(1996)).

In the bearing having the above-mentioned structure, when a shear loadis applied to the rotating shaft, the lubricating oil lubricatingbetween the rotating shaft and the journal part is pushed toward theenlarged diameter parts due to the occurrence of runout of the rotatingshaft then filled between the rotating shaft and the enlarged diameterparts. The lubricating oil filled between the rotating shaft and theenlarged diameter parts is pressed by the runout of the rotating shaftso as to be impregnated into the enlarged diameter parts. However, sincethe enlarged diameter parts are thickly formed, the lubricating oil isnot impregnated and remains between the rotating shaft and the enlargeddiameter parts to apply reaction forces to the rotating shaft. Therunout of the rotating shaft is suppressed by the reaction forces so asto prevent the deviation from the center of the rotating shaft withrespect to the bearing.

The bearing having the above-mentioned structure is very effective insuppressing deviation from the center of the rotating shaft.Accordingly, in a case where the conventional oil-impregnated sinteredbearing is used to support the rotating shaft, for example, if torque istransmitted to rotate the rotating shaft in a certain direction, a shearload is applied to the rotating shaft. However, when the shear load isvery large and the rigidity of the rotating shaft is not sufficientlyhigh, the rotating shaft is deflected due to the shear load and therotating shaft is rotated while an axis thereof is inclined in a bearingbody. Therefore, there is a possibility of becoming the state (motionthat the rotating shaft scrapes out the inner surface of the bearing)where the surface of the rotating shaft does not correctly come intocontact with the friction surface of the bearing. In such a state, sincethe rotating shaft receives strong resistance, it is difficult to rotatethe rotating shaft, thereby not sufficiently functioning as a bearing.Moreover, if the state is repeated, it is also considered thatdurability of the rotating shaft and the bearing deteriorates.

In addition, when a shear load causing runout of the rotating shaft isremarkably large and a push-back action caused by the lubricating oilremaining between the rotating shaft and the enlarged diameter partsdoes not sufficiently function, the rotating shaft is supported whilethe axis of the rotating shaft is inclined in the bearing body. In thiscase, since the surface of the rotating shaft is pushed againstboundaries between the journal part and the enlarged diameter partsthereby coming in contact with the bearing body at a point. In thiscase, if the rotating shaft acts as described above, both end of thejournal part are scraped out and stress concentration occurs at thecontact portion between the rotating shaft and the bearing body. In thisway, if the stress concentration occurs, excessive abrasion andoverheating occurs from the contact portion. This phenomenon does notoccur as long as a push-back action caused by the lubricating oilremaining between the rotating shaft and the enlarged diameter partsfunction. However, when a large unexpected shear load is suddenlyapplied, there is a possibility that durability of the rotating shaftand the bearing deteriorates.

In the bearing, since taper angles (Angles formed by an inclined planeof the enlarged diameter parts with respect to a longitudinal directionof the bearing along an axis of the journal part, that is, alongitudinal direction of the rotating shaft supported by the bearing,and angles formed between an inner surface of the journal part andinclined planes of the enlarged diameter parts are the same) are set tovery small angles of 2° to 3°, a very high machining accuracy isrequired. If the taper angles are not set accurately, there is apossibility of not sufficiently exhibiting center deviation-suppressingaction for the rotating shaft.

DISCLOSURE OF THE INVENTION

The invention has been made to solve the above-mentioned problems, andit is an object of the invention to provide an oil-impregnated sinteredbearing which functions as a bearing and has a high durability even inthe case that the rotating shaft is deflected due to a shear loadapplied to the rotating shaft.

Furthermore, it is another object of the invention to provide anoil-impregnated sintered which does not damage rotating shaft and itselfand has a high durability even in the case that the rotating shaft isinclined in the bearing by a large shear load applied thereto.

In addition, it is a still another object of the invention to provide amethod of manufacturing an oil-impregnated sintered bearing whichexhibits center deviation-suppressing action of the bearingsatisfactorily by accurately forming a bearing hole in an intermediatecompletely sintered.

An oil-impregnated sintered bearing having the following structure isemployed to solve the problems. That is, according to a first aspect ofthe invention, an oil-impregnated sintered bearing includes a bearinghole formed in the bearing body made of a sintered metal to support arotating shaft by an inner surface thereof as a friction surface. Inthis case, the bearing hole includes a journal part that has a constantdiameter, and enlarged diameter parts that are provided on both sides ofthe journal part in the longitudinal direction thereof, respectively, soas to be connected with the journal part.

According to a second aspect of the invention, in the above-mentionedstructure, it is preferable that the enlarged diameter parts be providedon both sides of the journal part in the longitudinal direction thereof,respectively; a taper angle with respect to the longitudinal directionof one enlarged diameter part which is provided on one side of thejournal part and a taper angle with respect to the longitudinaldirection of the other enlarged diameter part which is provided on theother side of the journal part be equal to each other; and a lineobliquely extending along an inclined surface of one enlarged diameterpart is arranged parallel to a line obliquely extending along aninclined surface of the other enlarged diameter part and a distancebetween the lines is substantially equal to the diameter of the rotatingshaft.

According to a third aspect of the invention, in the above-mentionedstructure, it is preferable that a distance between the line obliquelyextending along an inclined surface of the enlarged diameter part andthe journal part facing the inclined surface of the enlarged diameterpart across the middle of the bearing body be substantially equal to thediameter of the rotating shaft

According to a fourth aspect of the invention, in the above-mentionedstructure, it is preferable that the taper angles with respect to thelongitudinal direction of the enlarged diameter part be 3° or less.

According to a fifth aspect of the invention, in the above-mentionedstructure, it is preferable that the enlarged diameter parts be formedstepwise so that the taper angles with respect to the longitudinaldirection of the enlarged diameter parts are different from each other,and it is preferable that the taper angle of one enlarged diameter partpositioned away from the journal part be larger than the taper angle ofthe other enlarged diameter part.

According to a sixth aspect of the invention, in the above-mentionedstructure, it is preferable that the enlarged diameter parts be formedso that the difference between the taper angles of adjacent enlargeddiameter parts is 3° or less.

According to a seventh aspect of the invention, a method ofmanufacturing an oil-impregnated sintered bearing which has a bearinghole formed in the bearing body made of a sintered metal to support arotating shaft, the bearing hole including a journal part of which aninner surface as a friction surface has a constant diameter and enlargeddiameter parts that are provided so as to be connected with the journalpart and are formed in a tapered shape having diameters to be enlargedtoward the tips thereof, includes a process of forming a bearing holethat includes the journal part having a constant diameter by pressing aninner circumferential surface of a cylindrical sintered body completelysintered, and a process of forming the enlarged diameter parts so as tobe connected with the journal part by re-pressing the innercircumferential surface of the cylindrical sintered body.

According to an eighth aspect of the invention, in the above-mentionedmethod, it is preferable that substantially cone-shaped press dies eachhaving a base having a diameter larger than the inner diameter of thesintered body be used for forming the enlarged diameter parts.

According to a ninth aspect of the invention, in the above-mentionedmethod, it is preferable that the press dies be simultaneously insertedfrom both sides of the sintered body, respectively, and the tips of thepress dies be pushed against the inner circumferential surface of thesintered body so as not to come into contact with each other.

According to a tenth aspect of the invention, an oil-impregnatedsintered bearing has a bearing hole formed in the bearing body made of asintered metal to support a rotating shaft, and the bearing holeincludes a journal part of which an inner surface as a friction surfacehas a constant diameter and enlarged diameter parts that are provided soas to be connected with the journal part and are formed in a taperedshape having diameters to be enlarged toward the tips thereof. In thiscase, the bearing hole that includes the journal part having a constantdiameter by pressing an inner circumferential surface of a cylindricalsintered body completely sintered is formed, and then the enlargeddiameter parts so as to be connected with the journal part byre-pressing the inner circumferential surface of the cylindricalsintered body is formed.

In the invention, if torque is transmitted to rotate the rotating shaftin a certain direction, a shear load is applied to the rotating shaftand the rotating shaft is deflected due to the shear load. When therotating shaft is deflected so as to be inclined in the bearing, thesurface of the rotating shaft does not come into contact with thejournal part but comes into contact with the enlarged diameter parts sothat the rotating shaft is rotated while contacting the enlargeddiameter parts as friction surfaces. The enlarged diameter partsthermally expands due to the pumping action and frictional heat causedby rotation of the shaft to lubricate a friction surface, and exhibitsfunction as a bearing. That is, even though the rotating shaft isrotated while the rotating shaft is deflected and the axis thereof isinclined in the bearing body 1, the surface of the rotating shaft comesinto contact with the enlarged diameter parts to obtain the same actionas that of the conventional oil-impregnated bearing.

In addition, when the magnitude of torque transmitted to the rotatingshaft is different, the amount of deflection varies in proportion to themagnitude of torque and the oblique angle of the rotating shaft is alsovaried in the bearing. In the invention, when a relatively small torqueis transmitted to rotate the rotating shaft, the surface of the rotatingshaft comes into contact with the enlarged diameter parts having smalltaper angles and function as a bearing is exhibited as described above.Furthermore, when a large torque is exerted to rotate the rotatingshaft, the surface of the rotating shaft comes into contact with theenlarged diameter parts having large taper angles and exhibits functionsas a bearing. The enlarged diameter parts may be formed so as tocorrespond to the magnitude of torque.

Moreover, when a shear load causing runout of the rotating shaft isremarkably large and a push-back action caused by the lubricating oilremaining between the rotating shaft and the enlarged diameter partsdoes not sufficiently function, the rotating shaft is supported whilethe axis of the rotating shaft is inclined in the bearing body. In thiscase, in the invention, since the surface of the rotating shaft does notscrape out both ends of the journal part and is pushed against theenlarged diameter parts by a line, stress concentration does not occurat the contact portion. Accordingly, excessive abrasion and overheatingdoes not occur.

In the invention, the bearing hole including a journal part having aconstant diameter is formed by pressing an inner circumferential surfaceof a cylindrical sintered body completely sintered, and then theenlarged diameter parts are formed so as to be connected with thejournal part by re-pressing the inner circumferential surface of thecylindrical sintered body. Therefore, it is possible to accurately formangles between an inner surface of the journal part and the inclinedplanes of the enlarged diameter parts.

Both a process of manufacturing the bearing hole that includes a journalpart having a constant diameter by pressing an inner circumferentialsurface of a cylindrical sintered body completely sintered, and aprocess of manufacturing the enlarged diameter parts so as to beconnected with the journal part by re-pressing the inner circumferentialsurface of the cylindrical sintered body are referred to as sizingprocess for improving dimensional accuracy. However, in the sizingprocess, the bearing hole that includes a journal part and has aconstant diameter is formed first, and then the enlarged diameter partsare formed on the basis of the bearing hole. Therefore, positions of theenlarged diameter parts are accurately adjusted with respect to thejournal part whereby angles between the inner surface of the journalpart and the inclined planes of the enlarged diameter parts are alsoformed accurately.

In the invention, since truncated-cone shaped press dies each having abase having a diameter larger than the inner diameter of the sinteredbody are used for forming the enlarged diameter parts, it is possible toform the enlarged diameter parts with a high accuracy. The enlargeddiameter parts are formed to be copied from the conical surfaces of thepress dies. However, since portions of walls of the bearing body, whichare flared outward by pressing the bearing hole having an originalconstant diameter, are slightly pushed toward the journal part, theinner surface of the journal part rises. Accordingly, the inner diameterof the journal part decreases. In this case, since the press dies areformed in a conical shape, the rising inner surface of the journal partbecomes the enlarged diameter parts and boundaries between the journalpart and the enlarged diameter parts are very accurately formed withoutirregularities.

In the invention, the press dies are simultaneously inserted from bothsides of the sintered body, respectively, and the tips of the press diesare pushed against the inner circumferential surface of the sinteredbody so as not to come into contact with each other. For this reason,the inner surface of the journal part rises uniformly, and the diameterof the journal part is constant at any position thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view taken along an axis of a rotating shaftshowing an oil-impregnated sintered bearing according to a firstembodiment of the invention.

FIG. 2 is a view showing the entire outline of the rotating shaftinclined in the bearing.

FIG. 3 is a view showing the entire outline of a mechanism in which arotating shaft is supported at two points by the oil-impregnatedsintered bearings of FIG. 1.

FIG. 4 is an enlarged view of principal parts showing a state in whichthe oil-impregnated sintered bearing comes into contact with therotating shaft.

FIGS. 5A-5C are views sequentially illustrating a sizing process of themanufacturing processes of the oil-impregnated sintered bearing of FIG.1.

FIGS. 6A-6C are views sequentially illustrating a sizing process of themanufacturing processes of the oil-impregnated sintered bearing of FIG.1.

FIG. 7 is a view illustrating a shape of the bearing to be varied by thesizing.

FIG. 8 is a cross-sectional view showing an oil-impregnated sinteredbearing according to a second embodiment of the invention.

FIG. 9 is an enlarged view of principal parts showing a state in whichthe oil-impregnated sintered bearing comes into contact with therotating shaft.

FIG. 10 is a view showing the entire outline of a mechanism in which arotating shaft is supported at two points by the oil-impregnatedsintered bearings of FIG. 8.

FIG. 11 is a cross-sectional view taken along an axis of a rotatingshaft showing an oil-impregnated sintered bearing according to a thirdembodiment of the invention.

FIGS. 12A-12C are views sequentially illustrating a sizing process ofthe manufacturing processes of the oil-impregnated sintered bearing ofFIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

A first embodiment of an oil-impregnated sintered bearing according tothe invention will be described with reference to FIGS. 1 to 4.

An oil-impregnated sintered bearing (hereinafter, simply referred to asa bearing) shown in FIG. 1 includes a bearing hole 3 formed in thebearing body 1 made of a sintered metal to insert a rotating shaft 2therethrough. The bearing hole 3 has a circular cross section in a planeorthogonal to a longitudinal axis O of the rotating shaft 2, and isprovided with a journal part 3 a and enlarged diameter parts 3 b and 3c. The journal part 3 a is provided roughly at the center of the bearingbody 1 and has a diameter slightly larger than the diameter of therotating shaft 2. In this case, the journal part 3 a has a constantdiameter at any position in the longitudinal direction thereof.Furthermore, the enlarged diameter parts 3 b and 3 c are provided onboth sides of the journal part 3 a in the longitudinal direction thereofso that the enlarged diameter parts 3 b and 3 c are connected with bothends of the journal part 3 a, respectively. In this case, the enlargeddiameter parts 3 b and 3 c are formed in a tapered shape having aconstant diameter to be enlarged toward the tips thereof. Angles (taperangles) θ₁ formed between inclined planes of the enlarged diameter parts3 b and 3 c and an inner surface (or axis O of the rotating shaft 2) ofthe journal part 3 a parallel to the axial direction of the bearing body1 are set to 3° or less. In addition, in FIG. 1, the θ₁ is exaggeratedlyshown so as to be shown clearly.

When the bearing body 1 is seen from a cross section taken along theaxis O of the rotating shaft 2 (see FIG. 1), two enlarged diameter parts3 b and 3 c having a journal part 3 a therebetween are formed so as tosatisfy the following: a line L1 a which obliquely extends along aninclined surface of one enlarged diameter parts 3 b toward the center ofthe bearing body 1 is arranged parallel to a line L1 b which obliquelyextends along an inclined surface of the other enlarged diameter parts 3c positioned at an opposing corner with respect to the one enlargeddiameter parts 3 b toward the center of the bearing body 1, and adistance d1 between the line L1 a and the line L1 b is slightly largerthan a diameter D of the rotating shaft 2 and is substantially equal tothe inner diameter of the journal part 3 a.

Inner wall parts 4 of the bearing body 1 forming the enlarged diameterparts 3 b and 3 c have a higher sintering density than an inner wallpart 5 thereof forming the journal part 3 a, that is, gas cavitiesremaining on the surfaces and inside of the inner wall parts 4 aresmaller and less than those of the inner wall part 5. The difference indensity occurs in the respective parts of the bearing body 1 byadjusting pressure applied to the respective parts thereof in acorrection process performed after a sintering process.

The bearing having the above-mentioned structure is used in the state inwhich the bearing body 1 is impregnated with lubricating oil and therotating shaft 2 is inserted into the bearing hole 3. FIG. 3 shows anexample of a mechanism in which a rotating shaft 2 is supported at twopoints by the bearings. In the mechanism, a spiral gear 2 a is formed onthe circumferential surface of the rotating shaft 2, and both ends ofthe rotating shaft 2 are supported by the bearings. The mechanism isconstructed so that a spiral gear 5 rotated by a driving device (notshown) is engaged with the spiral gear 2 a of the rotating shaft 2 torotate the rotating shaft 2. Although the rotating shaft 2 is actuallynot deflected as much as shown in FIG. 3, the rotating shaft 2 isexaggeratedly shown to clarify the point of the description.

When a relatively small torque is transmitted to rotate the rotatingshaft 2, a shear load transmitted to the rotating shaft 2 is also smalland runout or deflection does not occur on the rotating shaft 2.Therefore, the surface of the rotating shaft 2 comes into contact withthe journal part 3 a, and the rotating shaft 2 is supported by thejournal part 3 a serving as a friction surface. Since the journal part 3a thermally expands due to pumping action and frictional heat caused byrotation of the rotating shaft 2, lubricating oil comes out of theinside of the bearing body 1 to lubricate the friction surface.

When a remarkably large torque is exerted to rotate the rotating shaft2, a shear load exerted on the rotating shaft 2 becomes large and therotating shaft 2 is deflected (two-dot chain line in FIG. 3). As shownin FIG. 4, the rotating shaft 2 is inclined in the bearing so that theaxis o is inclined. However, the surface of the rotating shaft 2 doesnot come into contact with not the journal part 3 a but comes intocontact with the enlarged diameter parts 3 b and 3 c to be supported bythe enlarged diameter parts 3 b and 3 c serving as friction surfaces.Since the enlarged diameter parts 3 b and 3 c thermally expand like thejournal part 3 a due to the pumping action and frictional heat caused byrotation of the rotating shaft 2, lubricating oil comes out of theinside of the bearing body 1 to lubricate the friction surfaces.

In addition, when a large torque is exerted to rotate the rotating shaft2, a shear load exerted on the rotating shaft 2 is large. Accordingly,strong runout occurs in the rotating shaft 2 to generate deviation fromthe center thereof. In this case, since the runout occurs in therotating shaft 2, the lubricating oil lubricating between the rotatingshaft 2 and the journal part 3 a is extruded to the one enlargeddiameter part 3 b and the other the enlarged diameter part 3 c.Therefore, the lubricating oil is filled between the rotating shaft 2and the enlarged diameter part 3 b, or between the rotating shaft 2 andthe enlarged diameter part 3 c. The lubricating oil filled between therotating shaft 2 and the enlarged diameter parts 3 b and 3 c ispressurized by the runout of the rotating shaft 2 so as to be pushedagainst enlarged diameter parts 3 b and 3 c. However, since the enlargeddiameter parts 3 b and 3 c are thickly formed, the lubricating oil isnot impregnated into the enlarger diameter parts and remains between therotating shaft 2 and the enlarged diameter parts 3 b and 3 c to applyreaction forces to the rotating shaft 2. The runout of the rotatingshaft 2 is suppressed by the reaction forces so as to prevent thedeviation from the center of the rotating shaft 2 with respect to thebearing.

However, when a shear load applied to the rotating shaft 2 is remarkablylarge and a push-back action caused by the lubricating oil remainingbetween the rotating shaft 2 and the enlarged diameter parts 3 b and 3 cdoes not sufficiently function, the rotating shaft 2 is supported whilethe axis of the rotating shaft 2 is inclined in the bearing body 1. Inthis case, as shown in FIG. 2, since the surface of the rotating shaft 2comes into contact with the enlarged diameter parts 3 b and 3 c not by apoint but by a line, respectively, stress concentration does not occurbetween the rotating shaft 2 and the bearing body 1. Therefore,excessive abrasion and overheating does not occur between the rotatingshaft 2 and the bearing body 1.

In the bearing, although the rotating shaft 2 is deflected due to theshear load and thus rotated while the axis O is inclined in the bearing,the surface of the rotating shaft 2 comes into contact with the enlargeddiameter part 3 b during the rotation of the rotating shaft 2, whereby afunction of a conventional oil-impregnated bearing is obtained.Accordingly, the function serving as a bearing does not deteriorate anddeterioration of durability does not occur.

Processes of manufacturing the above-mentioned bearing will be describedwith reference to FIGS. 5 to 7.

Respective processes which include mixing raw powder, forming andsintering are performed. Thereafter, sizing is performed. The sizingincludes two processes that have a process of forming the bearing hole 3having the journal part 3 a and a process of forming the enlargeddiameter parts 3 b and 3 c on both sides of the journal part 3 a.

As shown in FIGS. 5A to 5C, a die 10 that has a cylindrical hole 10 a, afirst core rod 11 that has a round bar shape and can be inserted intothe hole 10 a from below with clearances, a first upper punch 12 thatcan be fitted into the hole 10 a from above and has a simple annular tipsurface, and a first lower punch 13 that can be fitted into the hole 10a from below and has a simple annular tip surface are used in theprocess of forming the bearing hole 3 having the journal part 3 a.

The first core rod 11 has two different diameters at the tip and basethereof, and the outer diameter of the base thereof, which has a largerdiameter than the tip thereof, is substantially equal to the innerdiameter of a sintered body W. The first core rod 11 can be insertedinto and be extracted from the first lower punch 13. The die 10 is fixedin place, and the core rod 11, the first upper punch 12, and the firstlower punch 13 are driven by driving devices (not shown).

Furthermore, as shown FIG. 5A, the first lower punch 13 is fitted intothe hole 10 a of the die 10, and the core rod 11 is inserted into thehole 10 a through the first lower punch 13. Then, the sintered body W isput into the hole 10 a from above the die 10. The sintered body W isdisposed in the hole 10 a so that the tapered tip of the core rod 11 isinserted into the sintered body W.

Next, as shown FIG. 5B, the first upper punch 11 is fitted into the hole10 a to strongly push down the sintered body W. The pushed-down sinteredbody W is interposed between the first upper punch 11 and the firstlower punch 13 so as to be pressed from above and below by the firstupper punch 11 and the first lower punch 13 to be slightly pushed andcontracted. In addition, the outer surface of the sintered body W ispushed against the inner surface of the hole 10 a to be corrected in asmooth cylindrical shape, and the outer surface of core rod 11 is pushedagainst the sintered body W to correct the inner surface of the sinteredbody W in a smooth cylindrical shape (a bearing hole 3, which includes ajournal part 3 a and has a constant diameter, is formed in the sinteredbody W).

When the correction is completed, the first upper punch 12 is extractedfrom the hole 10 a. Subsequently, as shown in FIG. 5C, the first lowerpunch 13 is pushed up and then the corrected sintered body W is ejectedfrom the hole 10 a.

As shown in FIGS. 6A to 6C, a die 20 having a hole 20 a whose diameteris substantially equal to the outer diameter of the sintered body W, asecond core rod 21 that has a round bar shape and can be inserted intothe hole 20 a from above with clearances, a third core rod 22 that has around bar shape and can be inserted into the hole 20 a from below withclearances, a second upper punch 23 that can be fitted into the hole 20a from above and has a simple annular tip surface, and a second lowerpunch 24 that can be fitted into the hole 20 a from below and has asimple annular tip surface are used in the process of forming theenlarged diameter parts 3 b and 3 c on both sides of the journal part 3a.

The outer diameters of the second and third core rods 21 and 22 arelarger than the inner diameter of the sintered body W, and the tips 21 aand 22 a of the second and third core rods 21 and 22 are truncated-coneshaped press dies. Both tips 21 a and 22 a thereof have the samedimension, and diameters of the bases 21 b and 22 b thereof are largerthan the inner diameter of the sintered body W. The outer diameter ofthe tip surfaces 21 a and 22 a are smaller than the inner diameter ofthe sintered body W. The second core rod 21 can be inserted into orextracted from the second upper punch 23, and the third core rod 22 canbe inserted into or extracted from the second lower punch 24.

The die 20 is fixed in place, and the second and third core rods 21 and22, the second upper punch 23, and the second lower punch 24 are drivenby driving devices (not shown).

First, as shown FIG. 6A, the second lower punch 24 is fitted into thehole 20 a of the die 20, and the third core rod 22 is inserted into thehole 20 a through the second lower punch 24. Moreover, the second upperpunch 23 having the second core rod 21 inserted therein is put in astandby state above the die 20. Then, the sintered body W in which abearing hole 3 having a journal part 3 a is completely formed is putinto the hole 20 a from above the die 20.

Next, as shown FIG. 6B, the second upper punch 23 is fitted into thehole 20 a in synchronization with the second core rod 21 to push downthe sintered body W. The pushed-down sintered body W is interposedbetween the second core rod 21 and the third core rod 22 so as to bepressed by the second core rod 21 and the third core rod 22. In thiscase, driving distances and shapes of the tips of the core rods 21 and22 are determined so that tip surfaces 21 c and 22 c of the core rodsdoes not come into contact with each other.

The tips 21 a and 22 a of the core rods come into contact with openings,respectively, which are provided on both ends of the bearing hole 3formed by the previously performed sizing process, and then are insertedinto the bearing hole 3 so as to be guided in the longitudinal directionof the bearing hole 3. The sintered body W is corrected by pushingintruding conical surfaces of the tips 21 a and 22 a of the core rodsagainst the bearing hole 3 (the enlarged diameter parts 3 b and 3 c areformed on both sides of the journal part 3 a). In this case, sinceportions corresponding to the enlarged diameter parts 3 b and 3 c arere-pressed by the conical surfaces of the tips 21 a and 22 a, sinteringdensity of the portions corresponding to the enlarged diameter parts 3 band 3 c becomes high and the difference in density occurs between thejournal part 3 a and the above-mentioned portions.

When the correction is completed, the second core rod 21 and the secondupper punch 23 are extracted from the hole 20 a, subsequently, as shownin FIG. 6C, the second lower punch 24 is pushed up and then thecorrected sintered body W is ejected from the hole 20 a.

As mentioned above, in the sizing process, the bearing hole 3 thatincludes a journal part 3 a and has a constant diameter is formed first,and then the enlarged diameter parts 3 b and 3 c are formed on the basisof the bearing hole 3. Therefore, positions of the enlarged diameterparts 3 b and 3 c are accurately adjusted with respect to the journalpart 3 a and angles (taper angles) θ₁ between the inner surface of thejournal part 3 a and the inclined planes of the enlarged diameter parts3 b and 3 c are formed very accurately.

In addition, if the second and third core rods 21 and 22 which are pressdies each having truncated-cone shaped tips are used in forming theenlarged diameter parts 3 b and 3 c, as shown in FIG. 7, the enlargeddiameter parts 3 b and 3 c are formed so as to be copied from theconical surfaces of the tips 21 a and 22 a of the core rods. However,since portions (X portions in FIG. 7) of walls of the bearing body 1,which are flared outward by pressing the bearing hole 3 having anoriginal constant diameter using the conical surfaces of the tips 21 aand 22 a, are pushed toward the journal part 3 a, the inner surface ofthe journal part 3 a rises. Accordingly, the inner diameter of thejournal part 3 a decreases. In this case, since the tips 21 a and 22 aof the core rods are formed in a conical shape, the rising inner surfaceof the journal part 3 a becomes the enlarged diameter parts 3 b and 3 c,which are pushed against the conical surfaces of the tips 21 a and 22 a.Furthermore, boundaries between the journal part 3 a and thesubstantially enlarged diameter parts 3 b and 3 c are very accuratelyformed without irregularities. Moreover, the hardness of a portionbecoming the journal part 3 a increases by pressing the portion becomingthe journal part 3 a in advance. Accordingly, even though the core rodsare inserted into both ends of the bearing body 1, deformation caused byswelling does not occur on the inner surface of the journal part 3 a anda smooth cylindrical shape is maintained.

In addition, the second and third core rods 21 and 22 are inserted fromboth sides of the sintered body W at the same time, respectively, andthe tips 21 a and 22 a of the core rods are pushed against the innercircumferential surface of the sintered body W so as not to come intocontact with each other. Thereby, the inner surface of the journal part3 a rises uniformly as shown in FIG. 4, and the diameter of the journalpart 3 a is constant at any position thereof. Next, a second embodimentof the oil-impregnated sintered bearing according to the invention willbe described with reference to FIGS. 8 to 10.

A bearing shown in FIG. 8 includes a bearing body 11 that is made of asintered metal and has a bearing hole 13 formed therein. The bearinghole 13 has a circular cross section in a plane orthogonal to thelongitudinal axis O of the rotating shaft 2, and is provided with ajournal part 13 a, first enlarged diameter parts 13 b and secondenlarged diameter parts 13 c. The journal part 13 a is provided roughlyat the center of the bearing body 1 and has a diameter slightly largerthan the diameter of the rotating shaft 12. In this case, the journalpart 13 a has a constant diameter at any position in the longitudinaldirection thereof. Furthermore, the first enlarged diameter parts 13 bare provided on longitudinal opposite sides of the journal part 13 a sothat the first enlarged diameter parts 13 b are connected with both endsof the journal part 13 a, respectively. In this case, the first enlargeddiameter parts 13 b are formed in a tapered shape having constantdiameters to be enlarged toward the tips of the first enlarged diameterparts 13 b. Moreover, the second enlarged diameter parts 13 c areprovided on longitudinal opposite sides (further outside of the firstenlarged diameter parts 13 b) of the first enlarged diameter parts 13 bso that the second enlarged diameter parts 13 c are connected with bothends of the first enlarged diameter parts 13 b, respectively. In thiscase, the second enlarged diameter parts 13 c are formed in a taperedshape having constant diameters to be enlarged toward the tips of thesecond enlarged diameter parts 13 c.

The first enlarged diameter parts 13 b and the second enlarged diameterparts 13 c are formed stepwise so that taper angles between the firstenlarged diameter parts 13 b and the inner surface (or an axis of therotating shaft 2) of the journal part 13 a parallel to the axialdirection of the bearing body 1 are different from taper angles betweenthe second enlarged diameter parts 13 c and the inner surface thereof.In this case, the taper angles θ₂ of the second enlarged diameter parts13 c positioned away from the journal part 13 a are larger than thetaper angles θ₁ of the first enlarged diameter parts 13 b. The taperangles θ₁ of the first enlarged diameter parts 13 b are set to 3° orless, and the taper angles θ₂ of the second enlarged diameter parts 13 care set so that differences between the taper angles θ₂ of the secondenlarged diameter parts 13 c and the taper angles θ₁ of the firstenlarged diameter parts 13 b adjacent to the second enlarged diameterparts 13 c are 3° or less.

When the bearing body 11 is seen from a cross section taken along theaxis O of the rotating shaft 2 (see FIG. 8), two second enlargeddiameter parts 13 c positioned further outside of the first enlargeddiameter parts 13 b are formed so that a distance d2 between a line L2 aand a line L2 b is larger than a diameter D of the rotating shaft 2 andis substantially equal to the inner diameter of the journal part 13 a.In this case, the line L2 a extends along an inclined surface of onesecond enlarged diameter part 13 c toward the center of the bearing body11, and the line L2 b which extends along an inclined surface of theother second enlarged diameter part 13 c positioned at an opposingcorner with respect to the one second enlarged diameter parts 13 ctoward the center of the bearing body 11.

In the bearing, when the magnitude of torque transmitted to the rotatingshaft 2 is different, the amount of deflection varies in proportion tothe magnitude of torque and the oblique angle of the rotating shaft 2 isalso varied in the bearing. When a relatively small torque istransmitted to rotate the rotating shaft 2, as shown in FIG. 9, thesurface of the rotating shaft 2 comes into contact with the firstenlarged diameter parts 13 b having small taper angles and function as abearing is exhibited as described above. Furthermore, when a largetorque is exerted to rotate the rotating shaft 2, as shown in FIG. 9,the surface of the rotating shaft 2 comes into contact with the secondenlarged diameter parts 13 c having large taper angles and exhibitsfunctions as a bearing.

In this way, even when the oblique angle of the rotating shaft 2 is alsovaried in the bearing, the surface of the rotating shaft 2 comes intocontact with any one of the first enlarged diameter parts 13 b and thesecond enlarged diameter parts 13 c to obtain the same action as that ofthe conventional oil-impregnated bearing. Therefore, the functionserving as a bearing does not deteriorate and deterioration ofdurability does not occur.

In addition, in the present embodiment, the first and second enlargeddiameter parts 13 b and 13 c of which taper angles are varied in twosteps are provided in the bearing body 1 as enlarged diameter parts.However, if the magnitude of torque transmitted to the rotating shaft 2varies in multiple steps, the enlarged diameter parts may be formed inmultiple steps so that the oblique angle of the rotating shaft 2corresponds to the magnitude of torque in every step.

In the bearing according to the present embodiment, the following usageis also considered. Even though the magnitude of torque transmitted tothe rotating shaft 2 is constant in a case where the rotating shaft 2 issupported at plural points by the bearings, the oblique angles of therotating shaft 2 are mutually different at the plural points as long asdistances between the bearings and a mechanism (for example, the spiralgear 5 described in the first embodiment) for transmitting torque to therotating shaft 2 are mutually different. In this case, although bearingshaving an enlarged diameter part matched with each of the oblique anglesmay be used separately, a plurality of bearings having different shapesshould be prepared which increases cost. Accordingly, if a bearinghaving the enlarged diameter parts, which are formed in multiple stepsso as to match each of the oblique angles, and is manufactured byemploying the bearing according to the present invention, only one kindof bearing is used for supporting the rotating shaft 2 at plural points.For this reason, it is possible to reduce cost by standardizing parts.

For example, as shown in FIG. 10, an oblique angle γ1 of the rotatingshaft 2 in a bearing that is positioned relatively away from a torquetransfer mechanism contacting rotating shaft 2, and an oblique angle γ2of the rotating shaft 2 in a bearing that is positioned relatively closeto the torque transfer mechanism are different from each other. When theboth oblique angles are compared with each other, the oblique angle γ2of the rotating shaft 2 in a bearing that is positioned relatively closeto the torque transfer mechanism is larger than the oblique angle γ1.Accordingly, if a bearing is manufactured so that the taper angles θ₁ ofthe first enlarged diameter parts 13 b according to the presentembodiment is set to match γ1 and the taper angles θ₂ of the secondenlarged diameter parts 13 c is set to match γ2 and then is provided ateach of the supporting points of the rotating shaft 2, it is possible toreasonably and smoothly support the rotating shaft 2 at any of thesupporting points with one type of bearing, that is, bearings of thesame shape.

Next, a third embodiment of the oil-impregnated sintered bearingaccording to the invention will be described with reference to FIG. 11.The same reference numerals as those of the first embodiment are givento components already described in the first embodiment, and thedescriptions thereof are omitted. In the bearing according to thepresent embodiment, an enlarged diameter part 3 b is provided on onlyone side of the journal part 3 a, and a chamfered portion 3 d isprovided on the other side of the journal part 3 a. Since the chamferedportion 3 d is generally provided in the bearing hole 3 in order toeasily insert the rotating shaft 2 into the bearing hole 3, thechamfered portion 3 d does not come into contact with the rotating shaft2 even though the rotating shaft 2 is displaced in any manner.

In addition, when the bearing body 1 is seen from a cross section takenalong the axis O of the rotating shaft 2 (see FIG. 11), the journal part3 a and the enlarged diameter part 3 b are formed so that a distance d2(corresponding the length of a normal line drawn from the line L1 a in aterminating portion of the journal part remotest from the enlargeddiameter portion 3 b) between the line L1 a and an inner wall surface ofthe journal part 3 a is slightly larger than a diameter D of therotating shaft 2 and is substantially equal to the inner diameter of thejournal part 13 a. In this case, the line L1 a obliquely extends alongan inclined surface of the enlarged diameter part 3 b toward the centerof the bearing body 1 and the inner wall surface of the journal part 3 afaces the inclined surface of the enlarged diameter part 3 b across themiddle of the bearing body 1. Also, in the present embodiment, an angle(taper angle) θ₁ formed between an inclined plane of the enlargeddiameter part 3 b and an inner surface (or axis O of the rotating shaft2) of the journal part 3 a parallel to the axial direction of thebearing body 1 is set to 3° or less.

In the bearing having the above-mentioned structure, when a shear loadapplied to the rotating shaft 2 is remarkably large and a push-backaction caused by lubricating oil remaining between the rotating shaft 2and the enlarged diameter part 3 b does not sufficiently function, therotating shaft 2 is supported while the axis of the rotating shaft 2 isinclined in the bearing body 1. However, since the surface of therotating shaft 2 comes into contact with the enlarged diameter part 3 bnot by a point but by a line, stress concentration does not occur at thecontact portion between the rotating shaft 2 and the bearing body 1.Therefore, excessive abrasion and overheating does not occur between therotating shaft 2 and the bearing body 1.

Processes of manufacturing the above-mentioned bearing will be describedwith reference to FIG. 12. In the meantime, since processes from mixingraw powder to sintering and sizing process of forming the bearing hole 3having the journal part 3 a are equal to those of the first embodiment,descriptions thereof are omitted.

As shown in FIGS. 12A to 12C, a die 30 including a hole 30 a that has aninner diameter substantially equal to an outer diameter of a sinteredbody W, a fourth core rod 31 that has a round bar shape and can beinserted into the hole 30 a from above with clearances, a third upperpunch 32 that can be fitted into the hole 30 a from above and has asimple annular tip surface, and a third lower punch 33 that can befitted into the hole 30 a from below and has a simple annular tipsurface are used in the process of forming the enlarged diameter part 3b on both sides of the journal part 3 a.

An outer diameter of the fourth core rod 31 is larger than the innerdiameter of the sintered body W, and a tip 31 b the fourth core rod 31is a truncated-cone shaped press die. The diameter of a base 31 bthereof is larger than the inner diameter of the sintered body W, and anouter diameter of the tip surface 31 c is smaller than the innerdiameter of the sintered body W. Accordingly, the fourth core rod 31 canbe inserted into or extracted from the third upper punch 32.

The die 30 is fixed in place, and the fourth core rod 31, the thirdupper punch 32, and the third lower punch 33 are driven by drivingdevices (not shown).

First, as shown FIG. 12A, the third lower punch 33 is fitted into thehole 30 a of the die 30. Moreover, the third upper punch 32 having thefourth core rod 41 inserted therein is stood by above the die 30. Then,the sintered body W in which a bearing hole 3 including a journal part 3a and the chamfered portion 3 d are completely formed is put into thehole 30 a from above the die 30.

Next, as shown FIG. 12B, the third upper punch 32 is fitted into thehole 30 a in synchronization with the fourth core rod 31 to push downthe sintered body W. The pushed-down sintered body W is interposedbetween the third core rod 31 and the third lower punch 33 so as to bepressed by the third core rod 31 and the third lower punch 33.

The tip 31 a of the fourth core rod 31 comes into contact with oneopening which is formed one end of the bearing hole 3 formed by thepreviously performed sizing process, and then are inserted into thebearing hole 3 so as to be guided in the longitudinal direction of thebearing hole 3. The sintered body W is corrected by pushingtruncated-cone shaped surfaces of the tip 31 a of the fourth core rod 31into the bearing hole 3 (the enlarged diameter part 3 b is formed on oneside of the journal part 3 a). In this case, since a portioncorresponding to the enlarged diameter part 3 b is re-pressed by thetruncated-cone surface of the tip 31 a, sintering density of the portioncorresponding to the enlarged diameter part 3 b becomes high and thedifference in density occurs between the journal part 3 a and theabove-mentioned portion.

When the correction is completed, the third upper punch 32 and thefourth core rod 31 are extracted from the hole 30 a, subsequently, asshown in FIG. 12C, the third lower punch 33 is pushed up and then thecorrected sintered body W is ejected from the hole 30 a.

As mentioned above, in the sizing process, the bearing hole 3 thatincludes a journal part 3 a and has a constant diameter is formed first,and then the enlarged diameter part 3 b is formed on the basis of thebearing hole 3. Therefore, a position of the enlarged diameter part 3 bis accurately adjusted with respect to the journal part 3 a and theangle (taper angle) θ₁ between the inner surface of the journal part 3 aand the inclined plane of the enlarged diameter part 3 b is formed veryaccurately.

On the other hand, every bearing according to the first to the thirdembodiment has a structure that prevents deviation from the center ofthe rotating shaft by providing difference in density in respectiveparts of the bearing body 1. However, the invention is not applied toonly an oil-impregnated sintered bearing that has the above-mentionedstructure, and can also be applied to an oil-impregnated sinteredbearing of which bearing body has constant sintering density.

INDUSTRIAL APPLICABILITY

According to the invention, even when a rotating shaft is deflected dueto a large shear load and the rotating shaft is rotated while an axisthereof is inclined in a bearing body, a surface of the rotating shaftcomes into contact with tapered enlarged diameter parts by a line. Forthis reason, stress concentration does not occur at the contact portionand excessive abrasion and overheating does not occur between therotating shaft and the bearing body. Therefore, the bearing and therotating shaft are damaged, not themselves, thereby obtaining a highdurability.

In addition, even when the magnitude of torque transmitted to therotating shaft is different and an oblique angle of the rotating shaftvaries in the bearing, the surface of the rotating shaft constantlycomes into contact with the enlarged diameter parts having the taperangles to obtain the same action as that of the conventionaloil-impregnated bearing as long as the enlarged diameter parts areformed in multiple steps so as to have taper angles matching the obliqueangle. Therefore, the above-mentioned effects are obtained.

Furthermore, according to a method of manufacturing the bearing of theinvention, a bearing hole that includes a journal part and has aconstant diameter is formed first, and then the enlarged diameter partsare formed on the basis of the bearing hole. For this reason, positionsof the enlarged diameter parts are accurately adjusted with respect tothe journal part included in bearing hole. Therefore, it is possible toaccurately form angles between an inner surface of the journal part andinclined planes of the enlarged diameter parts, and as a result,exhibits center deviation-suppressing action of the oil-impregnatedsintered bearing satisfactorily.

1. An oil-impregnated sintered bearing comprising: a bearing body madeof a sintered metal to support a rotating shaft by an inner surfacethereof as a friction surface, said bearing body having a bearing holetherein; wherein the bearing hole includes a journal part that has aconstant diameter, and enlarged diameter parts that are provided on bothsides of the journal part in the longitudinal direction thereof,respectively, so as to be connected with the journal part.
 2. Theoil-impregnated sintered bearing according to claim 1, wherein theenlarged diameter parts are provided on both sides of the journal partin the longitudinal direction thereof, respectively, a taper angle withrespect to the longitudinal direction of one enlarged diameter partswhich is provided on one side of the journal part and a taper angle withrespect to the longitudinal direction of the other enlarged diameterparts which is provided on the other side of the journal part, are equalto each other, and a line obliquely extending along an inclined surfaceof one enlarged diameter part is arranged parallel to a line obliquelyextending along an inclined surface of the other enlarged diameter part,and a distance between the lines is substantially equal to the diameterof the rotating shaft.
 3. The oil-impregnated sintered bearing accordingto claim 1, wherein a distance between the line obliquely extendingalong an inclined surface of the enlarged diameter part and the journalpart facing the inclined surface of the enlarged diameter part acrossthe middle of the bearing body is substantially equal to the diameter ofthe rotating shaft.
 4. The oil-impregnated sintered bearing according toclaim 1, wherein the taper angles with respect to the longitudinaldirection of the enlarged diameter parts are 3° or less.
 5. Theoil-impregnated sintered bearing according to claim 1, wherein theenlarged diameter parts are formed stepwise so that the taper angleswith respect to the longitudinal direction of the enlarged diameterparts are different from each other, and the enlarged diameter partspositioned farther from the journal part has a larger taper angle. 6.The oil-impregnated sintered bearing according to claim 5, wherein theenlarged diameter parts are formed so that the difference between thetaper angles of adjacent enlarged diameter parts is 3° or less.
 7. Amethod of manufacturing an oil-impregnated sintered bearing whichincludes a bearing body made of a sintered metal to support a rotatingshaft, the bearing body having a bearing hole formed therein, thebearing hole including a journal part of which an inner surface as afriction surface has a constant diameter and enlarged diameter partsthat are provided so as to be connected with the journal part and areformed in a tapered shape having diameters to be enlarged toward thetips thereof, comprising: forming a bearing hole that includes thejournal part having a constant diameter by pressing an innercircumferential surface of a cylindrical sintered body completelysintered; and forming the enlarged diameter parts so as to be connectedwith the journal part by re-pressing the inner circumferential surfaceof the cylindrical sintered body.
 8. The method of manufacturing anoil-impregnated sintered bearing according to claim 7, whereinsubstantially cone-shaped press dies each having a base having adiameter larger than the inner diameter of the sintered body are usedfor forming the enlarged diameter parts.
 9. The method of manufacturingan oil-impregnated sintered bearing according to claim 8, wherein thepress dies are simultaneously inserted from both sides of the sinteredbody, respectively, and the tips of the press dies are pushed againstthe inner circumferential surface of the sintered body so as not to comeinto contact with each other.
 10. An oil-impregnated sintered bearingwhich includes a bearing body made of a sintered metal to support arotating shaft, the bearing body having a bearing hole formed therein,the bearing hole including a journal part of which an inner surface as afriction surface has a constant diameter and enlarged diameter partsthat are provided so as to be connected with the journal part and areformed in a tapered shape having diameters to be enlarged toward thetips thereof, wherein the bearing hole that includes the journal parthaving a constant diameter by pressing an inner circumferential surfaceof a cylindrical sintered body completely sintered is formed; and theenlarged diameter parts so as to be connected with the journal part byre-pressing the inner circumferential surface of the cylindricalsintered body is formed.